1. Field of the Invention
The present invention relates to a two-color image forming apparatus using electrophotography.
First, it should be noted that the two-color image forming apparatus referred to by the invention includes not only a case using two kinds of toners with different colors, but also a case using toners having the same color but different properties. For example, the present invention can be applied to the case where with the same black color, the one toner is non-magnetic whereas the other toner is magnetic, namely, application of magnetic information to a part of the image is intended.
2. Description of the Related Art
A two-color image forming apparatus has been proposed in which after a photosensitive body has been charged, electrostatic latent images with three levels are formed on a photosensitive body by changing light exposure in three levels of no exposure, weak exposure and strong exposure in accordance with color information and they are developed using positive-charged toners and negative charged toners to form a two-color toner image on the photosensitive body.
Such a two-color image forming apparatus gives rise to fringe development in which a periphery of the image with one color is fringed with another color so that a clear image cannot be obtained.
The mechanism of this fringe development can be elucidated as follows.
FIGS. 2A and 2B are graphs for explaining the fringe development in which the distribution of the electric potential and electric field are shown with respect to positions on the surface of a photosensitive body after exposure.
As shown in FIG. 2A, the surface potential after exposure of the photosensitive body in the two-color image forming apparatus includes a charging potential Vca at a no exposure area, an intermediate potential Vw at a weak exposure area and a discharging potential Vda at a strong exposure area. At the area charged with the charging potential of Vca, first toners are developed in a normal development by a developing machine with a developing bias voltage Vb1 applied. On the other hand, at the area charged with the discharging potential of Vda, the second toners are developed in an inverted development by the developing machine with a developing bias voltage Vb2 applied. At the area charged with the intermediate potential of Vw, no toners are developed, thus providing a white area. However, as seen from FIG. 2B showing the surface electric field on the photosensitive body, the area with the intermediate potential Vw provides inverted electric fields due to the edge effect in the vicinities of the areas with the potentials of Vca and Vda because there are large differences between Vw and Vca, and between Vw and Vda. Toners with the opposite charging polarities are applied to the areas with the opposite electric fields. Therefore, assuming that the first toner is black and the second toner is red, the white areas on the periphery of the black image is developed in red and the white area on the periphery of the red image is developed in black. This is referred to as "fringe" development because the periphery of the black image appears as if it is fringed with red, and the periphery of the red image is fringed with black. Such color printing, which should not be essentially formed, makes the image unclear and leads to the result of recording erroneous information as erroneous printing. This problem must be solved.
The fringe development has a property that it is conspicuous as the development bias voltages Vb1 and Vb2 is close to the intermediate potential Vw of the area with weak exposure, and not conspicuous as the former is far from the latter. Therefore, using this property, in order to reduce the fringe development, it can be proposed to leave the developing bias voltages Vb1 and Vb2 from Vw. However, this reduces a difference between the developing bias voltages and the potentials of the image area with the toners applied, i.e. Vca-Vb1 and Vda-Vb2. As a result, the amount of toners developed in the inherent image areas is reduced to attenuate the image density. Namely, the fringe development can be reduced, but the inherent development itself will be attenuated. Accordingly, such. a proposal cannot solve the problem of the fringe development.
As another means for solving the fringe development, a technique of using a developer with low resistance has been proposed in JP-A-1-189664. This technique uses the development agent with low resistance to attenuate the edge effect so that the electric field on the periphery becomes low. However, the developer with too low resistance gave rise to a secondary problem that carriers are applied on the photosensitive body. The carriers applied on the photosensitive body provide a gap between a toner image on the photosensitive body and a sheet of paper in transfer. This reduces the strength of the electric field in transfer thereby to lead to poor copying of the toner image. In this case, a part of the character or image drops off with being transferred on the sheet of paper. Accordingly, it is difficult to use the second means of using the developer with low resistance in order to solve the fringe development.
Even when the developing bias voltage is combined with the resistance of the developer under the condition solving the fringe problem, the following problem occurs.
Since the property of the photosensitive body will change with elapse of time by use and the discharging wire of a charger will deteriorate, the intermediate potential changes inevitably. When the intermediate potential changes so that its difference from the developing bias voltage becomes small, the fringe development occurs.
The resistance of the developer changes with an environmental change, a change in a toner density, and a time-varying change of a carrier surface, etc. When the resistance of the developer changes so as to become high, e.g. the environment is placed in a low humidity atmosphere or the toner density becomes high, the fringe development will also occur.
Further, when the rotating speed of the developing roll of the developing machine is changed, the force of the developer rubbing the surface of the photosensitive body changes. This influences the fringe development.
As described above, the fringe development, which is affected by various causes, is difficult to solve by the prior art.